AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Electrolysis of molten ionic compounds: Students should be able to predict the products of the electrolysis of binary ionic compounds in the molten state.
A binary ionic compound contains a metal cation and a non-metal anion.
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A binary ionic compound contains a metal cation and a non-metal anion. In the molten state, its ions are free to move, so electrolysis decomposes it into its elements. To predict the products, identify the ions from the formula, then apply charge attraction: the positive metal ion goes to the negative cathode to form the metal; the negative non-metal ion goes to the positive anode to form the non-metal. For example, molten sodium chloride gives sodium at the cathode and chlorine at the anode. (HT only: At the cathode, positive ions gain electrons to form atoms. At the anode, negative ions lose electrons to form atoms). The method works for any binary molten ionic compound, including lead bromide and potassium iodide.
What earns the marks
- Identify the metal cation and non-metal anion from the compound's formula, for example MgCl₂ gives Mg²⁺ and Cl⁻.
- Predict that the metal cation is discharged at the negative cathode because opposite charges attract.
- Predict that the non-metal anion is discharged at the positive anode.
- Name the elements formed and state that the molten compound is decomposed into its elements.
- (HT only) Explain that metal ions gain electrons at the cathode (reduction) and non-metal ions lose electrons at the anode (oxidation).
Common mistakes and how to put them right
Swapping the electrode products.
Put it right by remembering that the metal always forms at the cathode and the non-metal at the anode in a binary molten compound.
Forgetting that some non-metal products are diatomic.
Put it right by writing Cl₂, Br₂ or I₂ rather than Cl, Br or I.
Using the compound's formula as the product, for example saying 'sodium chloride' forms at an electrode.
Put it right by naming the elements sodium and chlorine separately.
Exam tips
- Write the ions first, then draw or state the electrode each ion moves to before naming the product.
- For a formula such as Al₂O₃, use the charges Al³⁺ and O²⁻ to predict aluminium at the cathode and oxygen at the anode.
- (HT only) Remember OIL RIG to help identify oxidation at the anode and reduction at the cathode.
Try the lesson
The start of the Electrolysis of molten ionic compounds: Students should be able to predict the products of the electrolysis of binary ionic compounds in the molten state. lesson, made from checked teaching notes for this specification point.
Have a go first
Molten lead bromide is a binary ionic compound. What is produced at the negative electrode when molten lead bromide is electrolysed?
Listen
A binary ionic compound contains a metal cation and a non-metal anion. In the molten state, its ions are free to move, so electrolysis decomposes it into its elements. To predict the products, identify the ions from the formula, then apply charge attraction: the positive metal ion goes to the negative cathode to form the metal; the negative non-metal ion goes to the positive anode to form the non-metal. For example, molten sodium chloride gives sodium at the cathode and chlorine at the anode. HT only: At the cathode, positive ions gain electrons to form atoms. At the anode, negative ions lose electrons to form atoms). The method works for any binary molten ionic compound, including lead bromide and potassium iodide. A strong answer earns marks for these points. Identify the metal cation and non-metal anion from the compound's formula, for example MgCl₂ gives Mg²⁺ and Cl⁻. Predict that the metal cation is discharged at the negative cathode because opposite charges attract. Predict that the non-metal anion is discharged at the positive anode. Name the elements formed and state that the molten compound is decomposed into its elements. A common slip: Swapping the electrode products. Instead, remembering that the metal always forms at the cathode and the non-metal at the anode in a binary molten compound. Exam tip: Write the ions first, then draw or state the electrode each ion moves to before naming the product.
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Key words
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binary ionic compound
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predicting products of electrolysis
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See every definition
- binary ionic compound
- A compound that contains a metal cation and a non-metal anion.
- predicting products of electrolysis
- Identify the ions from the formula, then apply charge attraction to name the element formed at each electrode.
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